ISEE is a NASA/ESA cooperative program consisting of three satellites intended to study the dynamic properties of Earth's magnetosphere and the solar wind in front of the magnetosphere (study the interaction of the interplanetary medium with the earth's immediate environment and to study the magnetosphere bow shock and magnetosheath in order to derive a better model of the interaction). Specific objectives of the mission were: 1)2)3)4)

• to investigate the solar-terrestrial relationships at the outermost boundaries of the Earth's magnetosphere

• to examine in detail the structure of the solar wind near the Earth and the shock wave that forms the interface between the solar wind and Earth's magnetosphere

• to investigate motions of and mechanisms operating in the plasma sheets

• to continue the investigation of cosmic rays and solar flare emissions in the interplanetary region near 1 AU.

The ISEE-1 and ISEE-3 spacecraft were the principal contributions of NASA, while ISEE-2 was built and managed by ESA. More than 100 investigators, representing most of the magnetospheric community, from 33 institutes were involved in the ISEE mission and its 32 instruments.

The three spacecraft carried a number of complementary instruments for making measurements of plasmas, energetic particles, waves, and fields. The mission thus extended the investigations of previous IMP (interplanetary Monitoring Platform) spacecraft.

The overall objectives were the observation of the near-Earth magnetosphere and its boundaries, better understanding of many phenomena, such as the Earth's bow shock, the magnetosheath and magnetopause, interactions between the tail and aurorae, and particle populations and flows in the tail.

ISEE-1 spacecraft:

ISEE-1 and ISEE-3 spacecraft are based on the IMP design pattern and were built by NASA as their main contribution to the IMS. TheISEE-1 spacecraft was spin-stabilized, had a mass of 340 kg (minisatellite) and a nominal power of 175 W. 6)7)8)9)

The ISEE-1 mission has also the designations of ISEE-A and Explorer 56.

The Explorer-class daughter spacecraft, ISEE-2, was part of the mother/daughter/heliocentric mission (ISEE-1, ISEE-2, ISEE-3). The mother/daughter portion of the mission consisted of two spacecraft (ISEE-1 and ISEE-2) with station-keeping capability in the same highly eccentric geocentric orbit.

The ISEE-2 minisatellite featured a spin-stabilized cylindrical bus with three deployed instrument booms. Strict measures were followed to eliminate interference from the spacecraft to some of the experiments: the entire exterior was made conductive to reduce potential difference to 1 V, the use of non-magnetic materials restricted ISEE's DC field to < 0.25 gamma at the magnetometer, and stringent limits were imposed on the electromagnetic radiation emitted by ISEE's interior. 10)

The EPS (Electric Power Subsystem) used silicon cells on cylindrical panels providing a power of > 100 W (65 W after 10 years); 27 W were required by the science payload. The EPS was supported by a NiCd battery which failed after 2 years (as predicted).

The ISEE-2 S/C was built by Dornier-System GmbH (prime contractor), heading the STAR consortium. The ISEE-2 spacecraft had launch mass mass of 166 kg (27.7 kg science payload) with a design life of 3 years.

Launch: ISEE-1 and ISEE-2 were launched in tandem (Delta-2914 launch vehicle, joint launch provided by NASA) on October 22, 1977 from Cape Canaveral into highly elliptical geocentric orbits. The satellites passed through the magnetosphere and into the magnetosheath during each orbit providing good coverage of all the magnetosphere features over the period of a year.

Orbit: HEO (Highly Elliptical Orbit) with an apogee of 23 RE (137,806 km) and a perigee of 1.04 RE (6,600 km), inclination = 28.76o. Both spacecraft penetrated into the interplanetary medium for up to 3/4 of an orbital period depending upon the time of year.

ISEE-1 and ISEE-2 were in almost coincident orbits around the Earth with periods of approximately 57 hours (3441 minutes), and their time separation in this orbit could be altered by maneuvering ISEE-2. These two spacecraft, separated by a variable distance (50 -5000 km) and with similar instrument complements, were able to break the space-time ambiguity inevitably associated with measurements by a single spacecraft on thin boundaries which may be in motion, such as the bow shock and the magnetopause. 11)

Status of the missions ISEE-1 and ISEE-2:

• Both spacecraft reentered the Earth's atmosphere on September 26, 1987 - completing 1517 orbits of the Earth (nearly 10 years of operational life was provided).

• ISEE-1 operated in a somewhat degraded mode due to the loss of one experiment and partial loss of four others of the total complement of 13 experiments. The battery failed due to normal wear-out after 4 1/2 years of service; however, this did not curtail operations due to the spacecraft being in a full-sunlight orbit.

• ISEE-2: No units failed, apart from the expected loss of its battery.

• Following the re-entry of ISEE-1 and -2 in 1987, a special effort was undertaken to archive at the NSSDC high-quality, high time resolution data about particles, fields and waves for specific time periods deemed to be of interest to the scientific community. For ISEE-1, these special archival periods are:

2) the period when ISEE-3 was in Earth's magnetotail (Oct. 15, 1982 - Dec. 25, 1983)

3) the "PROMIS" campaign period (March 29, 1986 - June 16, 1986).

Sensor complement of ISEE-1 and ISEE-2:

A special issue on instrumentation for the International Sun-Earth Explorer Spacecraft was provided in IEEE Transactions on Geoscience Electronics, Vol. GE-16, July 1978.

ANM/AND (Electrons & Protons)

PI: K. A. Anderson, UCB, the instrument is flown on ISEE-1 and -2. Objectives: Study of the varies energetic particle phenomena found in the Earth's magnetosphere, magnetopause, magnetosheath, bow shock, and upstream medium. Measurement over a wide range of energies, from ~1.5 to 300 keV for both electrons and protons.

The instrument was developed at UCB and consists of a pair of surface barrier semiconductor detector telescopes (one with a foil and one without a foil) and four fixed energy electric field particle analyzers. The analyzers are used to measure electrons and protons separately at 2 and 6 thousand electron volts.

PI: Louis A. Frank, University of Iowa. The instrument is also known by the designation FRM/FRD and is flown on ISEE-1 and ISEE-2. Objective: study of directional intensities of positive ions and electrons over a large solid angle. Energy range: 1 eV? E/Q ? 50 keV in 63 bands with 17% resolution. 13)

The instrument is a quadrispherical low-energy proton and electron differential energy analyzer (LEPEDEA), employing seven continuous channel electron multipliers in each of its two (one for protons and one for electrons). All but 2% of the 4? sr solid angle was covered for particle velocity vectors. A GM tube was also included, with a conical field of view of 40° full-angle, perpendicular to the spin axis. This detector was sensitive to electrons with E > 45 keV, and to protons with E > 600 keV. Instrument mass = 5 kg, power = 5 W.

RUM/RUD (Fluxgate Magnetometer Experiment):

PI: C. T. Russell, UCLA. The RUM/RUD fluxgate magnetometers were flown on ISEE-1 and ISEE-2. The overall objective was to obtain a quantitative understanding of the dynamic plasma and field environment of the Earth. 14)

Three NOL (Naval Ordnance Laboratory) ring core sensors in anorthogonal triad are enclosed in a flipper mechanism at the end of the magnetometer boom, 3 m from the skin of the spacecraft on ISEE l, and 2 m on ISEE 2. The flipper mechanism is actuated by heating a bimetallic strip which rotates the sensor from one stable spring-held position through 90o to a second position. During a "flip left" operation sensor which is initially anti parallel to the spin axis in the flip position, is rotated into the spin plane to look in the direction opposite spacecraft rotation. Sensor 3 is rotated from the spin plane looking in the direction of spacecraft rotation to a direction anti parallel to the spacecraft spin axis. A flip takes about 4 min at room temperature in vacuum, and requires about 5 W.

Both the ISEE l and 2 magnetometers were turned on a few hours after launch and have operated continuously since that time except for brief periods during interference testing. The only operational anomalies have been a couple of status changes of the ISEE 2 instrument which were not commanded from the ground. These both occurred during the first two weeks and have not reoccurred. The flippers have been exercised every five days on both spacecraft for a total of over 50 flips to date with no evidence of aging. 15)

The instrument has two commandable ranges of ±256 ? and±8192 ? with an accuracy of 0.025%.

FPE (Fast Plasma Experiment):

PIs: S. J. Bame, Los Alamos Scientific Lab, G. Paschmann, MPI Garching. Identical fast plasma experiment (FPE) systems were placed on the ISEE-1 and ISEE-2 spacecraft. Three electrostatic analyzers (with 90o spherical section) provide electron and proton measurements. Each instrument uses a divided secondary emitter system to intercept the analyzed particles. ISEE-1 carries also a solar wind experiment (SWE) to measure solar wind ions with high resolution. The SWE is composed of two 150Â spherical section analyzers using the same set of plates. The two acceptance fans are tilted with respect to each other so that 3D characteristics of the ion distributions can be derived. 16)

The experiment consists of the WIM instrument (Wide Angle Particle Spectrometer and a Heavy Ion Telescope) on ISEE-1 and the KED instrument (five sensor systems mounted at various angular positions with respect to the S/C spin axis) on ISEE-2.

- Protons: 20 keV - 2 MeV in 8 channels, in 16 channels on ISEE-1

- Electrons: 20 keV - 1.2 MeV in 8 channels, in 16 channels on ISEE-1

- Protons: 20 keV - 2 MeV in 12 channels on ISEE-2

- Electrons: 20 keV - 300 keV (to 1.2 MeV for 90o unit) on ISEE-2

GUM/GUD (Plasma Wave Investigation):

PI: D. A. Gurnett, University of Iowa. GUM/GUD is flown on ISEE-1 and ISEE-2. Objective: Study of wave/particle interaction in the Earth's magnetosphere and in the solar wind. The instrument on ISEE-1 uses three electric dipole antennas with lengths of 215 m, 73.5 m, and 0.6 m for the electric field measurements, and a triaxial search coil antenna for magnetic field measurements.

The ISEE-2 instrument uses two electric dipoles with lengths of 30 m and 0.6 m, and a single-axis search coil antenna for magnetic field measurements. The ISEE-2 plasma wave instrumentation consists of a 16 channel spectrum analyzer covering the frequency range from 5.62 Hz to 31.1 kHz and a wide-band waveform receiver with the capability of making waveform measurements in selected frequency ranges up to 2.0 MHz. 18)19)

Magnetic field levels

10 - 100 kHz (3 axis, 16 channels)

Electric field levels

10 Hz - 10 kHz (3 axis, 12 channels)

Sweep frequency spectrum

10 kHz - 200 kHz (128 steps), analysis of the electric field signals

Table 2: GUM/GUD parameters

HEM (VLF Wave Propagation Experiment)

PI: R. A. Helliwell, Stanford University; the instrument is flown on ISEE-1. Objective: Study of VLF-wave-particle interactions in the magnetosphere (note: VLF = Very Low Frequency in the 10 - 30 kHz range). A second goal is the determination of the effects upon energetic particles in the magnetosphere of electrical power transmission line radiation. 20)

The instrument setup consists of three separate elements:

- a broadband VLF receiver on ISEE-1

- a broadband VLF transmitter located at Siple station in the Antarctic

- ground stations in the Antarctic and Canada

During the IMS (International Magnetospheric Study), the ISEE-1 spacecraft has been an important component of the VLF wave-injection experiments for studying interactions between coherent VLF waves and energetic particles. The coherent waves are injected into the magnetosphere by ground-based transmitters such as the Siple Station, Antarctica, and those of the Omega navigation network.

EGD (Solar Wind Ion Experiment):

PIs: E. Egidi, G. Moreno, CNR Frascati, Italy; the instrument is flown on ISEE-2, it has also the designation of SWE (Solar Wind Ion Experiment). Objective: Study of the transient phenomena in the solar wind to obtain a spatial gradients of the interplanetary plasma. The instrument measures the flow directions and energy spectra of the positive ions in the solar wind. Two modes of operation are provided, one concentrates on high angular resolution and the other on high energy resolution. The main region of interest for this instrument is outward from and including the magnetopause.

The instrument is based on two identical hemispherical electrostatic energy selectors for the measurement of positive ions in two different energy windows.

The double probe, floating potential instrumentation on ISEE-1 is producing reliable direct measurements of the ambient DC electric field at the bow shock, at the magnetopause, and throughout the magnetosheath, tail plasma sheet and plasmasphere. In the solar wind and in middle latitude regions of the magnetosphere spacecraft sheath fields obscure the ambient field under low plasma flux conditions such that valid measurements are confined to periods of moderately intense flux. Initial results show: 21)

• a) that the DC electric field is enhanced by roughly a factor of two in a narrow region at the front, increasing B, edge of the bow shock

• b) that scale lengths for large changes in E at the sub-solar magnetopause are considerably shorter than scale lengths associated with the magnetic structure of the magnetopause

• c) that the transverse distribution of B-aligned E-fields between the outer magnetosphere and ionospheric levels must be highly complex to account for the random turbulent appearance of the magnetospheric fields and the lack of corresponding time-space variations at ionospheric levels.

HOM (Low Energy Cosmic Ray Experiment):

PI: Dieter K. Hovestadt, MPI Garching, Germany. The instrument is flown on ISEE-1 and ISEE-3. Objective: Measurement of elemental abundances, charge state composition, energy spectra, and angular distributions of energetic ions in the energy range of 2 keV/charge to 80 MeV/nucleon, and of electrons between 75 - 1300 keV. The instrument consists of three sensor systems: 22)

- ULECA is an electrostatic deflection analyzer, its energy range from about 3 to 560 keV/charge

- the ULEZEQ sensor consists of a combination of an electrostatic deflection analyzer and a thin-window proportional counter. The energy range is 0.4 MeV/nucleon to 6 MeV/nucleon. Objective: collection of composition data in the trapped radiation zone.

- study of the quasi-static electric field over a dynamic range of 0.1 - 200mV/m

- study of wave electric fields at frequencies <1000 Hz with a sensitivity < 1 µV/m (Hz)1/2 at all frequencies

- study of plasma density and temperature

Measurements are made of the potential difference between a pair of 8 cm diameter vitereous carbon spheres which are mounted on the ends of wire booms and are separated by 73.5 m in the spin plane of the satellite.

OGM (Fast Electron Spectrometer Experiment):

PI: K. W. Ogilvie, GSFC. The instrument is flown on ISEE-1. Objective: Study of three-dimensional plasma distribution in the solar wind, magnetosheath, outer magnetosphere, and near tail regions. Instrument provides three energy ranges: 7.5-512 eV, 11-2062 eV, and 109-7285 eV. Two channel electron multipliers are used at the output of each of six cylindrical electrostatic analyzers. The total mass of two sensors and a data processing unit is 4.9 kg and the power consumption is 3.5 W. Two hundred information bits/s telemetry rate is required. 24)25)

SHM (Ion Composition Experiment):

PI: R. D. Sharp, Lockheed, Palo Alto, CA. The instrument is flown on ISEE-1. Objective: Study of the composition of the hot magnetospheric plasma. Ion composition of the ring current, the plasma sheet, the plasmasphere, the magnetosheath, and the solar wind in order to establish the origin of the plasmas in the various regimes of the magnetosphere and to identify mass and charge dependent acceleration, transport, and loss processes. 26)

The instrument consists of two ion mass spectrometers which can be operated independently. The spectrometers point 5o above and 5o below the ISEE-1 spin plane. Measurement ranges: 1 AMU to > 150 AMU in 64 channels at each of 32 energy channels covering the energy per charge range from 0 to ~17 keV/e.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).